There are many situations in which it is necessary to separate a mixed granular or particulate material into granules or particles of different sizes, specific gravities or both. One example, in connection with which the present invention finds particular utility, is the remediation of indoor or outdoor firearm training facilities which have become saturated with lead from used bullets. In this saturated state, it can be unsafe to continue use of the firearm training facilities. In order to restore these sites to an unsaturated, usable condition, the lead bullets must be removed from the backstop material with which they are mixed, and then the backstop material may then be re-used, recycled or discarded. Different types of backstop material (e.g., sand, or granulated rubber), make it difficult to use any one type of remediation system at different facilities. Mechanical screening can, at least to some degree, be used to separate the mixture of backstop material and bullets into its component parts; however, since mechanical screening relies on size differences between the granules or particles to be separated, it is not capable of separating bullets and backstop material which are of the same or similar size.
Separation of used bullets from backdrop material allows for recycling of the lead, which requires a certain level of purity in the product to be recycled. By separating the lead bullets from similarly sized backstop material, the backstop material can be returned to the site for repeated usage. The lead bullets can then be removed from the site in a relatively pure form for recycling and reuse.
Air separation (also known as dry separation) provides a method for separating mixed granular or particulate materials into their component parts by relying on differences in the specific gravity (rather than size) of the granules or particles to be separated. The theory of air separation is well understood by those skilled in art. Briefly, air separation is carried out by allowing the mixed granular or particulate material to fall vertically by gravity across a horizontal stream or flow of air. Assuming that all of the granules or particles are of approximately the same size (and hence experience approximately the same drag force from the moving air), granules or particles of greater mass will be accelerated more slowly by the moving air than those of lesser mass. As a result, the heavier granules or particles will fall closer to the initial drop point than the lighter granules or particles. By positioning hoppers or receptacles at these locations, the heavier and lighter granules or particles can be collected and processed separately. Examples of air separators can be found in U.S. Pat. Nos. 775,965 and 2,978,103.
In theory, air separation provides a useful way to separate lead bullets from backstop material of similar size in an environmental remediation operation of the type described above. In reality, however, there are a number of problems with the prior art approaches. For example, prior art air separators are generally designed to operate with small size granular or particulate materials, but the backstop material at an indoor remediation site is generally much larger in granule size than typically encountered with outdoor granular materials. This can result in poor separation between the backstop material and lead bullets. Still another problem with existing types of air separators is the fact that the prior art separators are bulky, are by design more complicated, require large amounts of space, and are not mobile.
Yet another difficulty with the prior art separators is that use of them with firearm training facilities sometimes requires shutting down the facility to move all the saturated backstop material to the prior art separator where separation of the bullets from the backstop material occurs. The cleaned backstop material can then be reused in the same facility, or sold as scrap or for some other purpose.